Nascent product excitations in unimolecular reactions: The separate statistical ensembles method

Abstract

The unimolecular reaction of a vibrationally excited molecule having low rotational excitation often leads to nascent products in which the vibrational degrees of freedom appear ‘‘hotter’’ than the rotation, translation (R,T) degrees of freedom. We show that this can derive from parent vibrations being ‘‘hot’’ while parent rotations remain ‘‘cold,’’ since the parentage of product vibration is parent vibration, while product R,T excitations are obtained from parent vibration as well as rotation. Calculations are performed for reactions having loose transition states and no reverse barriers, in which an ensemble of 3N–6 degrees of freedom are used to equilibrate parent vibrations, thereby providing a statistical distribution of product vibrational excitations. For each set of product vibrational states, all R,T excitations are then apportioned statistically using the phase space theory of unimolecular reactions (PST). The results indicate that for those energies above reaction threshold (E‡) which exceed the lowest product vibrational energies, product vibrations are more excited than with PST, while product R,T excitations are less than with PST. These differences increase with E‡, and rotational distributions obtained using the separate statistical ensembles (SSE) method peak at low N″ relative to PST. When product vibrations are energetically inaccessible, SSE and PST are identical. The calculations are compared to nascent distributions from the unimolecular dissociation of monoenergetic NCNO, and the agreement is excellent.